Increasingly stringent legal requirements with regard to the permissible pollutant emissions of internal combustion engines installed in automobiles make it necessary to take various measures by which the pollutant emissions are reduced. Here a starting point is to reduce the pollutant emissions produced during the combustion process of the air/fuel mixture.
In order to achieve very good mixture preparation, fuel is increasingly metered under very high pressure. In the case of diesel combustion engines the fuel pressures are up to 2000 bar, for example. In the case of gasoline combustion engines the fuel pressures are up to approximately 200 bar. Injection valves having a piezoelectric actuator as the actuating mechanism are increasingly gaining acceptance for such applications. Piezoelectric actuators are distinguished by very short response times. Where applicable, such injection valves are therefore designed to meter fuel several times within one combustion cycle of a cylinder of the internal combustion engine.
A particularly good mixture preparation can be obtained if one or several pre-injections, also termed pilot injections, take place before a main injection, it being possible, if required, for a very small amount of fuel to be metered for the individual pre-injection. For these cases in particular, precise control of the injection valves is very important.
It is known from DE 196 52 807 A1 that in order to control a piezoelectrically operated fuel injection valve, in one control cycle of the actuator the charge of a capacitor that is charged up to the specified voltage is at least partially transmitted to the actuator during a specified charging time. Furthermore, the charging time of the following control cycle is varied by an absolute value stored in an area of an engine operating map assigned to this charging time and to the charging voltage of the actuator obtained in this charging time.
It is known from DE 100 63 080 A1 that the functional relationship between the electrical energy applied to the actuator and the actuator stroke is also temperature-dependent and that the temperature is to be taken into account during the control of the actuator. For this, the actuator controller has three temperature sensors which measure the cooling water temperature, the oil temperature and the fuel temperature and relays these to an evaluation unit which derives the actuator temperature therefrom. A characteristic unit inputs to a driver circuit a setpoint for the electrical charge to be applied to the actuator, on the basis of the actuator temperature, so that a constant stroke is set irrespective of the actuator temperature.